Polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) are the most commonly used binder materials for the anode and cathode of a lithium-ion battery or for the cathode of a lithium metal battery. However, both materials are not electrically or thermally conductive. Further, they are not an electro-active material; i.e. they are not capable of storing lithium when used as a binder in either the anode or the cathode. These features pose some undesirable consequences:                (1) The electrochemical reaction in a Li-ion battery is exothermic and thus the battery generates heat in both the charge and discharge cycle. Further, significant exothermic heat generation occurs in the Li-ion battery under abusive conditions, such as a short circuit, overcharging, over-discharging, and operation at high temperatures. The exothermic heat generation is attributed to a combination of effects, including the reaction of the PVDF in the electrodes with “lithiated” carbon, reaction of electrolyte with oxygen liberated due to decomposition of the cathode material, and breakdown of the electrode passivation layers. Low heat dissipation rates in the Li-ion battery can compromise the performance of the battery and may result in the release of combustible gasses at high temperatures, known as “thermal runaway”. Hence, it is critically important to have both the binder and the electrically conductive additive being thermally conductive as well.        (2) Due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 5%-20% (sometimes up to 50%), must be added into the electrode. Both the binder and the conductive additive are not an electrochemically active (electroactive) material. The use of a non-electroactive material means that the relative proportion of an electroactive material is reduced. For instance, the incorporation of 10% by weight of PVDF and 10% of carbon black in a cathode would mean that the maximum amount of the cathode active material (e.g., lithium cobalt oxide) is only 80%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-170 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.        
Clearly, an urgent need exists for a binder material that is conductive to both heat and electrons. Such a thermally conductive binder would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. If the binder is also electrically conductive, there would be reduced need or no need to have a separate conductive additive. Preferably, such a need for a separate additive is eliminated all together.
Thus, the present invention is directed to the provision of such a binder material for an electro-chemical cell that has effective heat dissipation capability and reduced electrical resistance during electrical charge and discharge of the battery.
The primary ingredient of the presently invented conductive binder is a graphene polymer. A graphene polymer is basically an aromatic, fused benzene ring-type of structure, which is substantially similar or identical to a graphene plane of a graphite or graphite oxide material. In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds. The graphene polymer may contain a small amount (typically<25% by weight) of non-carbon elements, such as hydrogen, fluorine, and oxygen, which are attached to an edge or surface of the graphene plane. The 2-D hexagonal carbon structure may be characterized in several essentially equivalent ways: (a) by the total number of carbon atoms in the planar structure, (b) by the number of carbon hexagons along each of the two directions (e.g., a and b or x- and y-direction) of a two-dimensional polymer structure, (c) by the sizes in the two directions (length and width), or (d) by the molecular weight of a 2-D polymer structure.
It may be noted that multiple graphene polymer units (i.e., several fused benzene ring units or carbon hexagon structure units) may stack up to form a multi-layer nano graphene platelet (NGP) or graphene nano-sheet when the graphene polymers precipitate out from a solution or suspension into a solid powder form. Hence, an NGP is a nanoscale platelet or sheet composed of one or more layers of a graphene plane, with a platelet thickness from less than 0.34 nm (single layer) to 100 nm (multi-layer, more typically <10 nm and most typically <2 nm). In the c-axis or thickness direction, these graphene planes may be weakly bonded together through van der Waals forces. The presently invented conductive binder typically and preferably contains mostly single-layer graphene structures while they are dispersed or dissolved in a liquid medium to form a graphene polymer suspension or solution. Such a graphene polymer suspension or solution is herein referred to as a binder precursor. The graphene binder polymer can contain a small amount of few-layer graphene platelets when the liquid medium is removed.
Graphene sheets or graphene polymer units may be oxidized to various extents during their preparation, resulting in graphite oxide (GO) or graphene oxide. Hence, in the present context, graphene polymers preferably or primarily refer to those containing no or low oxygen content; but, they can include GO of various oxygen contents. Further, graphene polymers may be fluorinated to a controlled extent to obtain graphite fluoride polymers, which are also considered one type of the presently invented graphene polymer.
It is of significance to herein point out that nano graphene materials, including single-layer and multi-layer NGPs, were not known in the art to be a binder capable of bonding solid particles together. As a matter of fact, when nano graphene powder and solid particles of an electro-active material are simply mixed together without using a resin binder (e.g. PVDF), the resulting mixture would be a sand-like mass containing separate particles and having no coherent integrity. Quite surprisingly, when the graphene sheets are dissolved or dispersed in a properly selected liquid, the resulting graphene polymer solution or suspension becomes a precursor to a conductive binder. By mixing particles of an electroactive material in this precursor solution or suspension to form a slurry or paste and then removing the liquid from the resulting slurry or paste, the resulting solid mass is characterized by having graphene polymers well bonded to particles of the electro-active materials (e.g., lithium cobalt oxide particles as a cathode active material and carbon-coated graphite particles as an anode active material). The resulting solid mixture is of good structural integrity even though no other resin binder (e.g., PVDF or SBR) is used. This discovery was most surprising and was not taught in the prior art.
Further surprisingly, a precursor binder solution or suspension does not have to be prepared by mixing already-isolated nano graphene sheets, either single-layer or multi-layer, in a liquid medium. The precursor binder solution or suspension can be obtained by directly mixing exfoliated graphite (graphite worms), expanded graphite flakes, or isolated minute graphite crystallites in a properly selected liquid medium at a temperature for a desired period of time. A graphite worm is composed of weakly interconnected graphite flakes (thickness>100 nm) and/or nano graphene platelets (thickness<100 nm). The interconnections in graphite worms can then be broken up, via ultrasonication or mechanical shearing, to obtain separated or isolated graphite flakes (also referred to as expanded graphite flakes with a thickness>100 nm) or NGPs (thickness<100 nm). The graphite worms, when re-dispersed in an acid medium, can be gradually dissolved to form a graphene solution.
The processes for producing graphene materials, including single-layer and multi-layer NGPs, have been recently reviewed by the applicants [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. Basically, there are four different approaches that have been followed to produce NGPs. Their advantages and shortcomings are briefly summarized as follows:
Approach 1: Formation and Reduction of Graphite Oxide (GO) Platelets
The first approach entails treating a laminar graphite material (e.g., in most cases, natural graphite powder) with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO). The obtained GIC or GO is then subjected to exfoliation using either a thermal shock exposure or a solution-based graphene layer separation approach.
Technically, the acid-treated graphite is actually oxidized graphite or graphite oxide (GO), rather than pristine graphite. In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050° C.) for a short period of time (typically 15 to 60 seconds) to exfoliate the treated graphite. Typically, the exfoliated graphite oxide is then subjected to a further sheet or flake separation treatment using air milling, mechanical shearing, or ultrasonication in a liquid (e.g., water).
In the solution-based graphene separation approach, the GO powder is dispersed in water or aqueous alcohol solution, which is subjected to ultrasonication. Alternatively, the GO powder dispersed in water is subjected to some kind of ion exchange or purification procedure in such a manner that the repulsive forces between ions residing in the inter-planar spaces overcome the inter-graphene van der Waals forces, resulting in graphene layer separations.
In both the heat- or solution-induced exfoliation approaches, the resulting products are GO platelets that must undergo a further chemical reduction treatment to reduce (but normally not eliminate) the oxygen content. Typically even after reduction, the electrical conductivity of GO platelets remains much lower than that of pristine graphene. Furthermore, the reduction procedure often involves the utilization of undesirable chemicals, such as hydrazine. In some cases of solution-based exfoliation, the separated and dried GO platelets were re-dispersed in water and then cast into thin GO films. These films were exposed to a high temperature, high vacuum environment for de-oxygenation, but the resulting GO platelets were no longer dispersible in water or other solvents.
Approach 2: Direct Formation of Pristine Nano Graphene Platelets
Jang, et al. succeeded in isolating single-layer and multi-layer graphene structures from partially carbonized or graphitized polymeric carbons, which were obtained from a polymer or pitch precursor. Carbonization involves linking aromatic molecules or planar cyclic chains to form graphene domains or islands in an essentially amorphous carbon matrix. For instance, polymeric carbon fibers were obtained by carbonizing polyacrylonitrile (PAN) fibers to a desired extent that the fiber was composed of individual graphene sheets isolated or separated from each other by an amorphous carbon matrix. The resulting fibers were then subjected to a solvent extraction, or intercalation/exfoliation treatment. Graphene platelets were then extracted from these fibers using a ball milling procedure. Please refer to B. Z. Jang and W. C. Huang, “Nano-scaled graphene plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006) and B. Z. Jang, “Process for nano-scaled graphene plates,” U.S. patent application Ser. No. 11/442,903 (Jun. 20, 2006).
Mack, Viculis, and co-workers developed a low-temperature process that involved intercalating graphite with potassium melt and contacting the resulting K-intercalated graphite with alcohol, producing violently exfoliated graphite containing many ultra-thin NGPs. [J. J. Mack, et al., “Chemical manufacture of nanostructured materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005); L. M. Viculis, et al “A chemical route to carbon nanoscrolls,” Science, 299, 1361 (2003)]. The process must be carefully conducted in a vacuum or an extremely dry glove box environment since pure alkali metals, such as potassium and sodium, are extremely sensitive to moisture and pose an explosion danger. It is questionable if this process is easily amenable to the mass production of nano-scaled platelets. One major advantage of this process is the notion that it produces non-oxidized graphene sheets since no acid/oxidizer intercalation or a high temperature is involved.
The applicants disclosed a direct ultrasonication method capable of exfoliating and separating NGPs from various graphitic materials without subjecting the graphitic material to chemical intercalation or oxidation [A. Zhamu, et al, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Plates,” U.S. patent application Ser. No. 11/800,728 (May 8, 2007)]. The resulting NGPs are pristine graphene and are highly conductive, both thermally and electrically.
Approach 3: Epitaxial Growth and Chemical Vapor Deposition of Nano Graphene Sheets on Inorganic Crystal Surfaces
Small-scale production of ultra-thin graphene sheets on a substrate can be obtained by thermal decomposition-based epitaxial growth and a laser desorption-ionization technique.
Approach 4: The Bottom-Up Approach (Synthesis of Graphene from Small Molecules)
X. Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc. 130 (2008) 4216-1765] synthesized nano graphene sheets with lengths of up to 12 nm using a method that began with Suzuki-Miyaura coupling of 1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid. The resulting hexaphenylbenzene derivative was further derivatized and ring-fused into small graphene sheets. This is a slow process that thus far has produced very small graphene sheets.
Nano graphene materials prepared by the aforementioned processes have recently been found to exhibit exceptionally high thermal conductivity, high electrical conductivity, and high strength. As a matter of fact, single-layer graphene exhibits the highest thermal conductivity and highest intrinsic strength of all existing materials. However, graphene has not been known to be capable of serving as a binder capable of bonding solid particles together. It is most surprising for us to observe that graphene behaves like an adhesive that can hold electroactive particles together to form an electrode of good structural integrity, obviating the need to use a non-conductive, non-electroactive resin binder and a conductive additive.
Thus, it is an object of the present invention to provide a thermally and electrically conductive binder based on a graphene polymer, including (but not limited to) a relatively oxygen-free graphene polymer, an oxidized graphene polymer (GO polymer), and a graphene fluoride polymer. The binder may be used in an electrode of an electrochemical cell, such as a lithium battery or a supercapacitor.
Another object of the present invention is to provide an electrochemical cell electrode containing a graphene polymer as a binder without having to use a non-conductive binder resin and/or additional conductive additive.
A further object is to provide an electrochemical cell containing such an electrode.
It is another object of the present invention to provide a precursor to a graphene polymer binder. The precursor contains a graphene polymer, graphene oxide polymer, or graphene fluoride dissolved or dispersed in a liquid medium (e.g. an organic solvent or water) to form a graphene polymer-solvent solution or graphene polymer-liquid suspension. Also provided is a paste containing electroactive particles dispersed in such a precursor solution or suspension.